The commercial demand for high capacity rechargeable electrochemical cells is strong. Many areas, such as aerospace, medical devices, portable electronics, and automotive, would benefit from cells having higher gravimetric and/or volumetric capacities. Lithium ion technology has already provided substantial improvements in this regard. However, to date, the technology has been primarily constrained to low capacity graphite based negative electrodes. Graphite has a theoretical capacity of only 372 mAh/g during lithiation and its practical capacity is even lower.
Silicon, germanium, tin, and many other materials have been proposed as replacements of or additives to graphite of their high lithiation capacities. For example, the theoretical capacity of silicon is estimated to be about 4,200 mAh/g. However, many of these high capacity materials have not been widely adopted because of their poor cycle life performance, which generally results from substantial volumetric changes during lithiation. Silicon, for example, swells by as much as 400% when it is lithiated to its theoretical capacity. Volume changes of such magnitudes cause considerable stresses in high capacity active material structures and their solid electrolyte interphase (SEI) layers and typically result in mechanical fractures and pulverization of the electrode structures and significant capacity fading of the electrochemical cell.